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E1 - Ch 3: Cell Structure and Function in Microbiology: Prokaryotes and Eukaryotes

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Cell Structure and Function in Microbiology

Introduction

This study guide covers the fundamental differences between prokaryotic and eukaryotic cells, focusing on their structures, functions, and relevance to human health. Key vocabulary terms are highlighted, and major cell components are described in detail, with examples and comparisons relevant to microbiology.

Prokaryotic vs. Eukaryotic Cells

Definitions and Key Differences

  • Prokaryotes: Cells lacking a membrane-bound nucleus and organelles. Includes Bacteria and Archaea.

  • Eukaryotes: Cells with a membrane-bound nucleus and organelles. Includes fungi, algae, protozoa, plants, and animals.

Feature

Prokaryotes

Eukaryotes

Size

~1-10 μm

~10-100 μm

Nucleus

No (nucleoid region)

Yes (membrane-bound)

Organelles

Absent

Present

Cell Wall

Peptidoglycan (most)

Cellulose (plants), chitin (fungi), variable

Examples

Bacteria, Archaea

Fungi, algae, protozoa, plants, animals

Bacterial Capsules and Glycocalyx

Structure and Function

  • Capsule: Composed of polysaccharide (most common) or protein; firmly attached to cell surface.

  • Function: Protects bacteria from recognition by host immune system and desiccation.

  • Glycocalyx: Less organized than capsules; helps animal cells adhere and communicate.

Flagella, Fimbriae, and Pili

Motility Structures

  • Flagella: Long, whip-like structures for movement; rotate 360° in prokaryotes, back-and-forth in eukaryotes.

  • Functions: Movement toward/away from stimuli (taxis), chemotaxis, and environmental adaptation.

  • Fimbriae: Short, bristle-like protein fibers; important for adhesion and biofilm formation.

  • Pili: Longer than fimbriae; involved in conjugation (DNA transfer).

Prokaryotic Cell Walls

Structure and Composition

  • Peptidoglycan: Polymer of sugars (NAG and NAM) and peptide bridges; provides structural support and protection from osmotic shock.

  • Antibiotic Target: Many antibiotics disrupt peptidoglycan synthesis.

Gram-Positive vs. Gram-Negative Cell Walls

Feature

Gram-Positive

Gram-Negative

Peptidoglycan Layer

Thick

Thin

Teichoic Acids

Present

Absent

Outer Membrane

Absent

Present (contains LPS)

Gram Stain

Retains crystal violet (purple)

Does not retain (pink/red)

LPS (Endotoxin)

Absent

Present; can cause fever, shock

Equation for Peptidoglycan Structure:

Bacterial Cytoplasmic Membrane

Structure and Function

  • Phospholipid bilayer: Composed of lipids and associated proteins; selectively permeable.

  • Fluid mosaic model: Describes dynamic nature of membrane.

  • Functions: Energy harvesting, transport of molecules, maintaining concentration gradients.

Osmosis and Solutions

  • Isotonic: Equal solute concentration inside and outside cell.

  • Hypertonic: Higher solute concentration outside; water leaves cell.

  • Hypotonic: Lower solute concentration outside; water enters cell.

Bacterial Internal Structures

Cytoplasm and Organelles

  • Cytosol: Liquid portion of cytoplasm; contains cell's DNA.

  • Ribosomes: Non-membrane-bound organelles; site of protein synthesis (70S in prokaryotes).

  • Endospores: Dormant, highly resistant structures for survival in harsh conditions (e.g., Bacillus anthracis).

Eukaryotic Cell Structures

External Structures

  • Glycocalyx: Less organized than prokaryotic capsules; aids in cell-cell recognition and protection.

  • Cell Wall: Composed of polysaccharides (cellulose in plants, chitin in fungi).

Cytoplasmic Membrane

  • Phospholipid bilayer with embedded proteins and sterols.

  • Membrane rafts: Regions of lipids and proteins; control movement into/out of cell.

Motility Structures

  • Cilia: Shorter and more numerous than flagella; coordinated movement propels cells.

  • Flagella: Longer, whip-like structures; movement by undulation.

Membranous Organelles

  • Nucleus: Contains most of cell's DNA; surrounded by nuclear envelope.

  • Endoplasmic Reticulum (ER): Network for transport; rough ER (with ribosomes), smooth ER (without ribosomes).

  • Golgi Body: Processes and packages proteins and lipids.

  • Ribosomes: Larger than prokaryotic (80S vs. 70S); site of protein synthesis.

Key Scientists in Microbiology

  • Hans Christian Gram: Developed Gram stain procedure, crucial for bacterial classification.

  • Bruce Ivins: Associated with research on Bacillus anthracis (anthrax).

Summary Table: Major Cell Structures and Functions

Structure

Prokaryotes

Eukaryotes

Capsule/Glycocalyx

Capsule (polysaccharide/protein)

Glycocalyx (less organized)

Cell Wall

Peptidoglycan

Cellulose, chitin, variable

Flagella

360° rotation

Undulation

Fimbriae/Pili

Present

Absent

Ribosomes

70S

80S

Nucleus

Absent

Present

Endospores

Present (some)

Absent

Additional info:

  • Gram stain is a critical diagnostic tool for infectious diseases.

  • Endospores allow bacteria like Bacillus anthracis to survive extreme conditions, including mail transit.

  • Hypertonic environments (e.g., honey, pickles) inhibit bacterial growth due to osmotic pressure.

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